What is the most efficient way to dynamically crea

Question

I don't know how many class instances I will have from the get-go, so I need to create them dynamically, but I would also like to keep the code tidy and readable.

I was thinking of doing something like this:

names = ['Jon','Bob','Mary']

class Person():
    def __init__(self, name):
        self.name = name

people = {}
for name in names:
    people[name] = Person(name)

It works, but I can't seem to find any examples of people doing this online (though I didn't look much). Is there any reason I should avoid doing this? If so, why and what is a better alternative?

Answer 1

Use type(name, bases, dict)

From documentation:

Return a new type object. This is essentially a dynamic form of the class statement. The name string is the class name and becomes the name attribute; the bases tuple itemizes the base classes and becomes the bases attribute; and the dict dictionary is the namespace containing definitions for class body and becomes the dict attribute. For example, the following two statements create identical type objects:

>>> class X(object):
...     a = 1
...
>>> X = type('X', (object,), dict(a=1))

For your example:

>>> JonClass = type('JonClass', (object,), {'name': 'Jon'})
>>> jon_instance = JonClass()
>>> jon_instance.name
'Jon'
>>> type(jon_instance)
<class '__main__.JonClass'>

Answer 2

How about using a generator expression to create the dictionary?

people = dict((name, Person(name)) for name in names)

But besides this your solution is perfectly valid.

Answer 3

If you want to create class instances dynamically, which is exactly what you are doing in your code, then I think your solution looks perfectly fine and is a pythonic way to do so (although I have to say there are of course other ways). Just to give you some food for thought: you could register/store each new instance with the class like that:

class Person():
    people={}
    @classmethod
    def create(cls,name):
        person=Person(name)
        cls.people[name]=person
        return person
    def __init__(self, name):
        self.name = name

And if you are getting adventerous, you can try the same with metaclass, but I will leave that for your research :-)